1. Field of the Invention
The present invention relates to a driving power control device for a hybrid vehicle, which comprises an engine and a motor-generator as power sources, and an automatic transmission disposed between the power sources and driving wheels with an intervening torque converter having a lockup clutch, and which controls the degree of engagement of a lockup clutch during a regenerative operation.
2. Description of the Related Art
Conventionally, a hybrid vehicle is known in the art, in which an engine and a motor-generator are provided as driving sources, and a portion of the kinetic energy of the vehicle is converted, by the motor-generator, into electrical energy which is stored in a battery device (generally known as a regenerative process) so that fuel efficiency can be improved. In such hybrid vehicles, a hybrid vehicle is also known, in which a torque converter having a lockup clutch is provided between the driving sources and driving wheels, and transmission efficiency of the driving power is adjusted by the lockup clutch.
In the above-mentioned regeneration process, if the lockup clutch is connected to the motor-generator during regenerative process, the kinetic energy of the driving wheels can be directly transmitted to the motor-generator. However, because torque fluctuations in the power sources are also directly transmitted to the driving wheels, noise and vibration are generated, which leads to degradation of drivability. Therefore, it is important to control the engaging state of the lockup clutch so as to obtain high efficiency of regeneration while preventing the above problems.
To prevent the above problems, a control device has been proposed, which controls the oil pressure for a lockup clutch so that power from power sources is transmitted with slip during a regeneration process. For example, a control device is disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-170903, which comprises a control section for controlling the engaging state of a lockup clutch, and which performs a control operation while receiving feedback of the engaging state (oil pressure) of the lockup clutch so that maximum efficiency of regeneration can be obtained.
However, a hybrid vehicle is driven in various driving states (for example, with or without brake operation, with or without cylinder deactivation operation, etc.), and obtainable regenerated electrical power (the amount of regeneration) varies depending on the driving state. Therefore, when the driving state changes, the amount of regeneration may vary considerably. Because the engaging state (controlling oil pressure) of the lockup clutch required to obtain the amount of regeneration at times varies considerably, it is impossible to control the engaging state so as to track large variations by controlling the oil pressure for the lockup clutch merely by a feedback method as described above.